(1) Field of the Invention
The present invention relates to radio receivers and radio communication systems, and more particularly, to a radio receiver that performs reception control of radio signals and a radio communication system that performs communication control of a radio signal.
(2) Description of the Related Art
Generally, PSK (Phase Shift Keying) that changes the phase of a carrier wave is used in digital modulation. As compared to ASK (Amplitude Shift Keying), PSK has the smallest C/N (Carrier-to-Noise) for the same code error ratio.
Recently, the zero-IF (Intermediate Frequency) system has widely been used, in which the received radio signal and a locally oscillated signal of the same frequency as that of the received radio signal are mixed so that an intermediate frequency (zero IF) at which the central frequency fo of the received radio signal is shifted to a frequency of zero (fo=0). The zero-IF signal is then amplified.
FIG. 14 shows the concept of the zero-IF system. The left side of the figure shows an energy distribution of a received radio signal, and the right side thereof shows an energy distribution of the zero-IF signal. The horizontal axes denote the frequency, and the vertical axes denote power. The received radio signal is frequency-converted into the zero-IF signal, which has a band width of ±1 MHz about the center of f0=0 MHz.
The zero-IF system does not provisionally convert the received radio signal into the intermediate frequency but directly converts it into the baseband signal. Thus, the zero-IF system has advantages such that the channel selection filter can be implemented in the baseband and an image wave does not occur at the time of reception of spurious signals.
FIG. 15 shows a structure of a conventional radio receiver. A radio receiver 100 is a QPSK (Quadrature PSK) receiver that employs the zero-IF system, and has two split systems that respectively generate an I (In phase) signal and a Q signal (Quadrature phase).
The radio receiver 100 includes an RFA (Radio Frequency Amplifier) 101 and a frequency converter 110. The frequency converter 110 is made up of an LO (Local Oscillator) 111, a π/2 phase shifter 112 and mixers 113a and 113b. 
The I signal generating system that follows the frequency converter 110 is made up of a LPF (low-pass filter) 102a, an AMP (Amplifier) 103a, couplers (capacitor) 104a, 106a, a VGC (Voltage Gain Controller) 105a, an A/D (Analog-to-Digital) converter 107a, a DEM (Demodulator) 108a, and an AGC (Automatic Gain Controller) 109a. 
Similarly, the Q signal generating system is made up of a LPF 102b, an AMP 103b, couplers 104b, 106b, a VGC 105b, an A/D converter 107b, a DEM 108b, an AGC 109b. 
In operation, the RFA 101 amplifies the received radio signal. The LO 111 of the frequency converter 110 oscillates a local signal of the same frequency as that of the received radio signal. In the I-signal generating system, the mixer 113a mixes the signal that has been amplified by the RFA 101 and the local oscillation signal from the LO 111, and thus generates a zero-IF signal.
Then, the signal is filtered and gained by the LPF 102a and the AMP 103a, and the dc component thereof is then cut off by the coupler 104a. The voltage gain of the signal is set at the level that matches the dynamic range of the A/D converter 107a, and the remaining dc component thereof is cut off by the coupler 106a. 
The signal is converted into a digital signal by the A/D converter 107a, and is demodulated into the I signal by the DEM 108a. The level setting of the voltage gain of the VGC 105a is adjusted by feedback control from the AGC 109a. 
In the Q signal system, the oscillation frequency from the LO 111 is shifted by π/2 by the π/2 phase shifter 112. The mixer 113b mixes the amplified signal from the RFA 101 and the π/2-phase-shifted oscillation signal. This mixing results in the zero-IF signal. Then, the same process as that for the I signal is performed, so that the Q signal can be generated.
Here, it is impossible to apply the zero-IF signals that have been amplified by the AMPs 103a and 103b to the VGCs 105a and 105b without the couplers 104a and 104b, respectively.
This is because the VGCs 105a and 105b is required to have an amplification factor of one million times (120 dB) (a few μV to a few V). A variation in the operating point as small as a few μV may cause cutoff or saturation, such as variation resulting from variation in environment and such as variation in the receive electric field intensity or temperature and/or differences resulting from the non-linearity of the elements. The AGCs 109a and 109b feed back the above-mentioned variation, so that the variation in the operating point may be further emphasized.
Taking into consideration the above, the radio receiver 100 is designed so that the systems that follow the AMPs 103a and 103b use the couplers 104a, 104b, 106a and 106b for cutting off the DC components and are AC-coupled with a processing part of the following stage.
However, in the AC-coupled of the conventional radio receiver, it is preferable to cut off the DC components of the Zero-IF signal to suppress DC fluctuation. It is to be noted that there is a lot of information at the frequency of the zero-IF signal and frequencies close thereto. Therefore it is desired to pass such information to the processing part of the following stage as much as possible. This causes complexity in optimal design.
That is, if the cutoff frequency is lowered, the receiver will be considerably affected by dc fluctuation. In contrast, if the cutoff frequency is raised, energy of the low-frequency component is cutoff highly and, namely, information will be lost greatly.